With the recent rapid spread of the Internet, a large capacity, advancement, and economization of an access service system have been required, and a passive optical network (PON) has been studied as a means for realizing the large capacity, the advancement, and the economization. The PON is a communication system that achieves economization by sharing one station-side device and a portion of a transmission path among a plurality of subscriber devices using an optical multiplexer/demultiplexer based on an optical passive device.
Currently, an economical optical subscriber system in which a line capacity of 1 Gbps are shared among a maximum of 32 users using time division multiplexing (TDM), and a gigabit Ethernet passive optical network (GE-PON, Ethernet is a registered trademark) have been mainly introduced in Japan. Accordingly, a Fiber To The Home (FTTH) service has been provided at a realistic fee.
Further, in order to meet needs of larger capacity, a 10G-EPON (Ethernet passive optical network) in which a total bandwidth is 10 Gbps has been studied as a next generation optical subscriber system, and international standardization has been completed in 2009. This is an optical subscriber system that realizes a large capacity while using a transmission line portion such as an optical fiber that is the same as that of GE-PON by increasing a bit rate of a transceiver.
In the future, a large capacity exceeding the 10G-class such as high-resolution video services or ubiquitous services may be considered to be required, but there is a problem that practical realization through an simple increase in a bit rate of a transceiver from the 10G-class to the 40/100G-class is difficult due to an increase in system upgrade cost.
As a means for solving this problem, a variable wavelength WDM/TDM-PON in which variable wavelength capability is added to a transceiver, and time division multiplexing (TDM) and wavelength division multiplexing (WDM) are effectively combined so that a transceiver in the station-side device can be additionally installed according to a bandwidth request amount has been reported (for example, see Non-Patent Document 1).
A variable wavelength WDM/TDM-PON has recently attracted attention as a system in which gradual expansion of a total bandwidth or flexible load distribution can be realized according to needs of users, in Non-Patent Document 2. When the total bandwidth is gradually expanded, changing of an optical subscriber unit (OSU) to which an optical network unit (ONU) belongs due to load distribution is realized through wavelength switching in the optical network unit (ONU). FIG. 1 illustrates a variable wavelength WDM/TDM-PON system related to the present invention. The variable wavelength WDM/TDM-PON system related to the present invention includes a station-side subscriber accommodation device (Optical Line Terminal; OLT) 10 and a subscriber device (ONU) 20. The OLT 10 includes a dynamic wavelength bandwidth allocation circuit 101, a demultiplexing unit 106, and an OSU 107. The OLT 10 is connected to the ONU 20 by a PON topology of a point-to-multipoint configuration that uses an optical multiplexer/demultiplexer 11, an optical multiplexer/demultiplexer 12, and optical fibers 13, 14, 15 and 16. The optical multiplexer/demultiplexer 11 and the optical multiplexer/demultiplexer 12 are, for example, a power splitter or a wavelength router. The demultiplexing unit 106 of the OLT 10 is connected to a relay network 40.
The OLT 10 includes line card OSU#1 to OSU#m that transmit and receive a set λ1d,u to λmd,u of downstream wavelengths λ1d to λmd and upstream wavelengths λ1u to λmu, and the dynamic wavelength bandwidth allocation circuit 101. OSU#1 to OSU#m transmit and receive respective wavelength signals of the set λ1d,u to λmd,u of wavelengths transmitted from the ONU 20. ONU#1 to ONU#h, i.e., h units of ONUs 20, are connected to the OLT 10, and each ONU 20 performs transmission and reception using any one in the set λ1d,u to λmd,u of downstream and upstream wavelengths. The ONU 20 may perform transmission and reception using any one in the set λ1d,u to λmd,u of wavelengths according to an instruction from the OLT 10.
An upstream signal from an installed communication device of a user's home is input to each ONU 20 and transmitted as an upstream optical signal in an optical transceiver in the ONU 20. The upstream signal is multiplexed to one optical fiber 13 from a power splitter or a wavelength router on the ONU 20 side toward the OLT 10. Therefore, the OLT 10 calculates and controls a transmission time and a transmission duration of upstream signals transmitted by the respective ONUs 20 so that the upstream signals do not overlap. The upstream signals 1 to m received in OSU#1 to OSU#m are aggregated by the demultiplexing unit 106 in the OLT 10, multiplexed into one upstream signal, and transmitted to the relay network 40. On the other hand, a downstream signal from the relay network 40 to each ONU 20 is separated into downstream signals 1 to m directed to OSU#1 to OSU#m on the basis of destination information of the ONU 20 described in the downstream signal in the demultiplexing unit 106 and information of the OSU 107 to which the ONU 20 belongs. The separated downstream signals 1 to m are sent to the respective ONUs 20 at downstream wavelength λ1d to λmd respectively set in OSU#1 to OSU#m. The downstream signal is broadcast at a wavelength of each OSU 107, but since a transmission and reception wavelength of the ONU 20 is set to a transmission and reception wavelength of the OSU 107 to which the ONU 20 belongs, the ONU 20 selects information addressed to the ONU 20 from the signal at a reception wavelength, and the information is output from the ONU 20 to the communication device of the user's home.
The dynamic wavelength bandwidth allocation circuit 101 includes a dynamic wavelength and bandwidth assignment (DWBA) calculation unit 103, a switching instruction signal generation unit 102, a control signal transmission unit 104, and a request signal reception unit 105. In the dynamic wavelength bandwidth allocation circuit 101, the request signal reception unit 105 receives a signal including a bandwidth request transmitted from each ONU 20, via each OSU 107, and the DWBA calculation unit 103 calculates the transmission time and the transmission duration of the upstream data signal and the request signal allocated to each ONU 20 on the basis of the request. Then, in the dynamic wavelength bandwidth allocation circuit 101, the switching instruction signal generation unit 102 generates an instruction signal including the calculated transmission time and the calculated transmission duration, and the instruction signal is transmitted from the control signal transmission unit 104 to each ONU 20 via each OSU 107. Further, the DWBA calculation unit 103 manages connection information of the ONU 20 and the OSU 107 in a PON period. When the wavelength is switched, the demultiplexing unit 106 is instructed to change the destination OSU 107 of the downstream signal of the ONU 20 that has changed the wavelength.
FIG. 2 illustrates a configuration of the ONU 20. The ONU 20 includes a data reception unit 201, a data transmission unit 208, an upstream buffer memory 202, a downstream buffer memory 209, a destination analysis and selection reception unit 210, a frame transmission control unit 203, a frame assembly and transmission unit 204, a wavelength-tunable optical transceiver 205, a request bandwidth calculation unit 206, a request signal transmission unit 207, an instruction signal reception unit 211, and a wavelength switching control unit 212.
The upstream signal from the user is received by the data reception unit 201 and temporarily stored in the upstream buffer memory 202. The frame transmission control unit 203 transmits the upstream signal to the frame assembly and transmission unit 204 according to a transmission time and a transmission duration of the upstream signal specified by the instruction signal. The frame assembly and transmission unit 204 constitutes a frame format required to transmit a signal to the OLT 10 in a PON configuration and sends the resultant signal to the wavelength-tunable optical transceiver 205.
The wavelength-tunable optical transceiver 205 converts the optical signal at any one of the wavelengths λ1d,u to λmd,u designated in the wavelength switching control unit 212, and transmits the optical signal. The downstream signal from the OSU 107 is received by selecting a specified wavelength in the wavelength-tunable optical transceiver 205, and a destination of the downstream signal is analyzed and only the information addressed to the device itself is selected in the destination analysis and selection reception unit 210, and stored in the downstream buffer memory 209. The data transmission unit 208 transmits the information stored in the downstream buffer memory 209 to the user as the downstream signal.
The wavelength-tunable optical transceiver 205 receives an instruction signal from the OLT 10, converts the instruction signal into an electrical signal, and sends the electrical signal to the instruction signal reception unit 211. The instruction signal reception unit 211 analyzes instruction content of the instructing signal, and sends a switching destination wavelength and a switching execution instruction to the wavelength switching control unit 212 at a designated time if a wavelength switching instruction, a wavelength after switching, and a switching start time are included in the instruction signal. The wavelength switching control unit 212 switches the wavelength of the wavelength-tunable optical transceiver 205 according to the wavelength switching control.
Further, the OLT 10 receives information on a bandwidth requested by the ONU 20 from the ONU 20 and uses the information to allocate the bandwidth. There are various methods, such as a method in which an instruction is performed to transmit the information on the requested bandwidth to the OLT 10 using an instruction signal, and the ONU 20 describes the information on the requested bandwidth directed to the OLT 10 in a request signal according to the instruction. In this case, if the instruction signal reception unit 211 receives the instruction signal for requesting request signal transmission, the instruction signal reception unit 211 instructs the request signal transmission unit 207 to generate the request signal. The request signal transmission unit 207 instructs the request bandwidth calculation unit 206 to calculate the requested bandwidth. The request bandwidth calculation unit 206 measures the amount of data of an upstream signal stored in the upstream buffer memory 202, determines a required bandwidth amount on the basis of the data amount, and sends the requested bandwidth amount to the request signal transmission unit 207. The request signal transmission unit 207 generates a request signal in which the requested amount has been described, and sends the request signal to the frame transmission control unit 203.
The instruction signal for the OLT 10 to instruct the ONU 20 to transmit the requested bandwidth may also include information on a transmission start time and a transmission duration of the request signal. In this case, the instruction signal reception unit 211 sends the information on the transmission start time and the transmission duration of the request signal included in the instruction signal to the frame transmission control unit 203, and the time frame transmission control unit 203 sends the request signal to the frame assembly and transmission unit 204 at an instructed time and transmits the request signal to the OLT 10 via the wavelength-tunable optical transceiver 205. Further, the instruction signal transmitted from the OLT 10 includes a transmission start time and a transmission duration at and in which the ONU 20 transmits an upstream signal received from the user, to the OLT 10. The instruction signal reception unit 211 sends information on the transmission start time and the transmission duration of the upstream signal included in the instruction signal to the frame transmission control unit 203, and the frame transmission control unit 203 extracts a frame of the upstream signal from a buffer memory at an instructed time, sends the frame to the frame assembly and transmission unit 204 for the transmission duration, and transmits the frame to the OLT 10 via the wavelength-tunable optical transceiver 205.
Further, Patent Document 1 describes an optical communication system having a function of the ONU 20 selecting an OSU 107 to which the ONU 20 newly belongs and reconnecting to the OSU 107 when an OSU 107 fails as a function of improving reliability and availability of the OLT 10, and an optical communication abnormality recovering method. In Patent Document 1, a wavelength at which a newly reconnected OSU 107 performs transmission and reception can be held as a switching information table in the ONU 20 when an abnormality, such as downstream signal interruption due to failure of the OSU 107 to which the ONU currently belongs, occurs in a reception signal by utilizing wavelength switching that is characteristic of the variable wavelength WDM/TDM-PON, and the communication can be returned earlier by performing the wavelength switching and a connection operation when the ONU 20 detects the abnormality. Further, this switching destination auxiliary communication wavelength is held as a wavelength at which a different OSU 107 performs transmission and reception for each ONU 20. Accordingly, when any OSU 107 fails, the ONUs 20 reconnected to the auxiliary OSU 107 are distributed, and a high-speed reconnection process due to the distribution of a reconnection process of the OSU 107 can be achieved or traffic after reconnection can be distributed to the respective OSUs 107.
However, after the OSU 107 in which the abnormality has occurred is recovered by exchange or the like, a so-called switch-back work of reconnecting the ONU 20 reconnected due to abnormality occurrence to the initially used OSU 107 may be performed for return to a current state. In Patent Document 1, a form in which a auxiliary OSU 107 can be distributed is described, but when return to a current state is performed, it is necessary to trace back a history indicating which ONU 20 has moved to which OSU 107 and send a wavelength switching instruction to the OSU 107 for each recovered ONU 20. This is because when the number of ONUs 20 connected to an initially failed OSU 107 increases, a time or an operation required for a switch-back operation increases and this becomes a burden on a communication network operation. Therefore, in order to reduce an operation burden, it is necessary to have a means for simply performing work of switching the distributed and reconnected ONU 20 back to the recovered OSU 107.